The invention concerns a device for monitoring the transmission of power line carrier currents.
Nowadays, companies or other organizations distributing electrical power to each user of an electrical power distribution network are having to perform more and more service management operations and to provide users with more and more services.
In particular, it is vitally important for such distribution companies or organizations to perform these operations in a manner that is totally or almost totally transparent for users.
The concept of transparency or quasi-transparency encompasses, on the one hand, avoiding repetitive physical intervention on the users' premises, for minimum disturbance to their public, professional or private activities, and, on the other hand, avoiding installing additional physical media for communicating data indispensable to providing or managing the services previously referred to.
In the context of use of the above type of communication means, distribution organizations such as ELECTRICITE DE FRANCE in France have developed communicating customer interface (CCI) systems employing power line carrier currents.
The aim of using power line carrier currents is not to provide a very high bit rate enabling transmission of fixed or animated video images, but to the contrary to provide a generalized service to each user, if possible independently of the configuration of the low voltage network and the relative amount of energy consumed by the user concerned.
Thus at the current stage of development of CCI systems the available data bit rate is 300 bit/s. Depending on the applications concerned, this bit rate could be increased to 2 400 bit/s in the near future.
Data is transmitted in frames synchronized to the "atural" frequency of the AC line voltage, which is 50 Hz in Europe.
Data is transmitted by power line carrier currents in the form of frames comprising 42 bytes, each bytes being encoded by FSK modulation at different first and second frequencies. For low voltage electrical power distribution in particular, the first carrier frequency representative of a true logic value is 60.6 kHz and the second carrier frequency representative of a complemented value of the true logic value is 74 kHz. The modulation process is Spread Frequency Shift Keying (S-FSK).
For provision of services related to street-lighting, the modulation is performed in a frequency band with a center frequency of 86 kHz and the modulation process is frequency shift keying (FSK).
For the provision of services relating to the domestic life of users, i.e. "home automation" applications, the center frequency is 132.5 kHz and the modulation process is usually FSK. In the near future it will be possible to transmit data for this purpose at 2 400 bit/s.
The data frames transmitted, comprising 42 bytes, are followed by absence of any carrier for the duration of 3 bytes, constituting the minimum period of silence between two consecutive frames.
In normal operation, each CCI system incorporating a low voltage PLC (power line carrier) modem repeats frames it receives when it is not sending, with a given repetition credit, every 1.2 s. This mode of operation enables step by step transmission of data, from a management center to any CCI system installed on the network and vice versa, as shown in FIG. 1. The communication protocol is suited to propagation problems on the low voltage distribution network. The repetition of frames, with credits, exploits the synchronous operation of the various CCI systems in order to eliminate any risk of conflict. In a particular frame-time only one frame is present on the low voltage network. The system as a whole has a client-server architecture in which the concentrator in the high voltage to low voltage transformer substation is the master. It stores in memory a table for associating a number of credits (repeats) with each CCI system on a user's premises. The maximum credit of 7 corresponds to 8 repetitions of a frame. Any CCI system receiving an intelligible frame, whether it is addressed to it or not, repeats it in the next frame-time. If the frame is addressed to it, the CCI system concerned sends its response in the frame-time immediately following the 0 credit frame of the incoming message; otherwise, the CCI system waits to receive a new frame before repeating it. FIG. 1 shows operation for communication between the concentrator and the CCI interface with the value of the credit (2 in this case) indicated in each frame, "RECEPTION A" denoting correct reception of the "go" message and "RECEPTION B" denoting correct reception of the "return" message.
In all cases transmitting data, in particular frames, every 1.2 seconds under steady state conditions indicates satisfactory PLC transmission.
It is also totally compatible with assessing the transmission rate by ear. The fact that the low voltage PLC modem of a CCI system repeats data frames reaching it in normal operation or does not repeat them in the case of a fault is significant of its correct or incorrect operation.
At present an approved operative of the distribution organization responsible for installing or administering a CCI system has no simple means of monitoring its operation other than removing the cover of the device and observing the status of the light-emitting diodes. This purely visual inspection does not provide any information as to the intensity of the signals. Qualitative information about the signals can only be obtained by the use on site of costly and bulky equipment, such as an oscilloscope, a spectrum analyzer or the like.